Explosive materials are generally reactive substances which have a great amount of potential energy that can give an explosion accompanied by light, heat, sound and pressure. Among the explosive materials, nitro compounds like 2,4-dinitrotoluene (2,4-DNT), 2,4,6-trinitrotoluene (TNT), 1,3,5-tinitroperhydroxo-1,3,5-tetrazine (RDX), etc are some of the generally used components. Apart from being used as an explosive material, nitro compounds like TNT are hazardous to human health which can cause various health concerns like anemia, abnormal liver function, etc. Considering the hazardous nature of the nitro compounds in terms of both security concerns and as a pollutant, the need for the detection of these materials has been the primary importance.
Researchers all over the globe have been working on various techniques for the detection of explosive material. Some of the known techniques are metal detectors which generally detect metal based weapons, canines with their superior sensing capabilities, X-ray machines by analyzing the density of the materials, neutron activation where the explosive materials is bombarded with neutrons which gives its elemental composition, and so on. Apart from the above mention techniques, spectroscopic techniques especially fluorescence based chemo sensors, due to their high sensitivity and probability of using as a hand held devices for in-field detection have superior advantage. Generally, in this approach, the fluorescent materials on interaction with the explosives materials either turn-off or quenches the luminescent of the sensors which relies on an oxidative quenching mechanism. In detail, the sensor material plays the role of an electron donors and aromatic nitro compounds as an electron acceptor due to the presence of their electron withdrawing nitro groups. On excitation with photon, an electron is transferred from the sensor materials to the analyte, leading to oxidation of the excited state, thereby quenching the fluorescence of the sensors. Some of the generally used fluorescent materials are conjugated polymers, metal complexes, dendrimers, carbon nanotubes, and recently the use of metal organic framework (MOF) as the new generation of sensors materials for explosive detection.
Article titled, “Enhanced emission of ultra-small-sized LaF3:RE3+ (RE=Eu, Tb) nanoparticles through 1,2,4,5-benzenetetracarboxylic acid sensitization” by Suwen Li et. al in Nanoscale, 2012, 4, 5619-5626 reports that uniform, ultra-small-sized and well-water-dispersible LaF3 nanoparticles doped with trivalent rare earth (RE) ions (Eu3+ or Tb3+) have been synthesized by a simple, low temperature synthesis route. The nanoparticles, with sizes of about 3.2 nm (for those doped with Eu3+) and 3.0 nm (for those doped with Tb3+), are roughly spherical and monodisperse. 1,2,4,5-Benzenetetracarboxylic acid (labeled as BA) as sensitizer has been bonded to the surface of the nanoparticles, which can sensitize the emission of RE3+ in the LaF3 nanoparticles. The BA-LaF3:RE3+ (RE=Eu or Tb) nanoparticles have a broad absorption band in the UV domain, and show enhanced luminescence of RE3+ based on an energy transfer from BA ligands to RE3+ ions (i.e. the so-called “antenna effect”). Due to the dual protection of organic ligands (BA) and inorganic matrices (LaF3), BA-LaF3:RE3+ (RE=Eu or Tb) nanoparticles have longer excited state lifetimes than LaF3:RE3+ (RE=Eu or Tb) nanoparticles as well as lanthanide coordination polymers of BA.
Article titled, “Multifunctional inorganic-organic hybrid nanospheres for rapid and selective luminescence detection of TNT in mixed nitro aromatics via magnetic separation.” by Ma Y et. al in Talanta. 2013 Nov. 15; 116:535-40 reports rapid, sensitive and selective detection of 2,4,6-trinitrotoluene (TNT) in aqueous solution differentiating from other nitroaromatics and independent of complicated instruments is in high demand for public safety and environmental monitoring. In this work, via a simple and versatile method, LaF3:Ce(3+)—Tb(3+) and Fe3O4 nanoparticle-codoped multifunctional nanospheres were prepared through self-assembly of the building blocks. The luminescence of these nanocomposites was dramatically quenched via adding nitroaromatics into the aqueous solution. After the magnetic separation, however, the interference of other nitroaromatics including 2,4,6-trinitrophenol (TNP), 2,4-dinitrotoluene (DNT), and nitrobenzene (NB) was effectively overcome due to the removal of these coexisting nitroaromatics from the surface of nanocomposites. Due to the formation of TNT(−)—RCONH3(+), the TNT was attached to the surface of the nanocomposites and was quantitatively detected by the post exposure luminescence quenching. Meanwhile, the luminescence intensity is negatively proportional to the concentration of TNT in the range of 0.01-5.0 μg/mL with the 3σ limit of detection (LOD) of 10.2 ng/mL.
Article titled, “Energy transfer from benzoic acid to lanthanide ions in benzoic acid-functionalized lanthanide-doped CaF2 nanoparticles” by Jianshe Wang et. al in Applied Surface Science, Volume 257, Issue 16, 1 Jun. 2011, Pages 7145-7149 reports the preparation of benzoic acid-functionalized CaF2:Ln3+ (Ln=Eu or Tb) nanoparticles and their sensitized luminescence. First, to achieve sufficient proof for energy transfer from benzoic acid (BA) to lanthanide ions doped in nanoparticles, we employ Eu3+ as the microscopic probe and investigate the luminescent spectra of benzoic acid-functionalized CaF2:Eu3+ (BA-CaF2:Eu3+) nanoparticles. Next, to further reveal the difference between sensitized luminescence and common luminescence for Eu3+ doped in CaF2 nanoparticles, we study the emission spectra of BA-CaF2:Eu3+ nanoparticles excited at 286 nm and 397 nm, respectively. Finally, we analyze and compare the luminescent spectra of BA-CaF2:Tb3+ and CaF2:Ce3+, Tb3+ nanoparticles in detail.
Article titled, “Inkjet printing lanthanide doped nanorods test paper for visual assays of nitroaromatic explosives” by Liang Hong in Analytica Chimica Acta, Volume 802, 13 Nov. 2013, Pages 89-94 reports the inkjet printed polyethylenimine (PEI)-coated Ce, Tb co-doped NaGdF4 nanorods (NaGdF4:Ce/Tb NRs) onto common filter paper to construct test paper for visual and instant detections of a typical explosive 2,4,6-trinitrophenol (TNP). Polyethylenimine molecules not only facilitate the formation of uniform NaGdF4 nanorods but also provide specific recognized sites for TNP by the acid-base pairing interaction. The resultant TNP bound at the surface of PEI-coated NaGdF4:Ce/Tb NRs can strongly quench the phosphorescence with a remarkably high quenching constant by the charge transfer mechanism from NaGdF4:Ce/Tb NRs to TNP. By printing of the probe on a piece of filter paper, trace amounts of TNP can be visually detected by the appearance of a dark color against a bright green background under a UV lamp. This test paper can detect TNP as low as 0.45 ng mm−2 by the naked eye, which provides a potential application in the rapid, on-line detections of explosives.
Article titled, “Ligand-centered near-infrared luminescence from lanthanide complexes with chelating nitronyl nitroxide free radicals” by Christophe Lescop in Inorganic Chemistry, September 2000; 39(17), 3740-1 reports Lanthanum(III), europium(III), and gadolinium(III) complexes with chelating nitronyl nitroxide free radicals showing luminescence between 700 and 1000 nm.
Article titled, “Formation and Luminescence Phenomena of LaF3:Ce3+ Nanoparticles and Lanthanide-Organic Compounds in Dimethyl Sulfoxide” by Wei Chen et. al in The Journal of Physical Chemistry C, December 2009; 114(2) reports LaF3:Ce3+-doped nanoparticles synthesis at different temperatures in dimethyl sulfoxide (DMSO) by the chemical reaction of lanthanum nitrate hydrate and cerium nitrate hexahydrate with ammonium fluoride. The formation of Ce3+-doped LaF3 nanoparticles is confirmed by X-ray diffraction and high-resolution transmission electron microscopy. An intense emission at around 310 nm from the d-f transition of Ce3+ was observed from the LaF3:Ce3+ powder samples. However, in solution samples, the ultraviolet emission from Ce3+ is mostly absent, but intense luminescence is observed in the visible range from blue to red. Article titled, “Lanthanide-doped calcium phosphate nanoparticles with high internal crystallinity and with a shell of DNA as fluorescent probes in cell experiments” by Sussette Padilla Mondéjar et. al in J. Mater. Chem., 2007, 17, 4153-4159 reports Calcium phosphate nanoparticles prepared by precipitation and stabilized as colloids by coating with DNA. They were doped with europium or terbium during this precipitation (about 2.5 wt %) and showed good fluorescence in the visible part of the spectrum.
Article titled “Lanthanide Sensitization in II-VI Semiconductor Materials: A Case Study with Terbium(III) and Europium(III) in Zinc Sulfide Nanoparticles” by Prasun Mukherjee et. al in J. Phys. Chem. A, 2011, 115 (16), pp 4031-4041 reports the sensitization of luminescent lanthanide Tb3+ and Eu3+ cations by the electronic structure of zinc sulfide (ZnS) semiconductor nanoparticles. Excitation spectra collected while monitoring the lanthanide emission bands reveal that the ZnS nanoparticles act as an antenna for the sensitization of Tb3+ and Eu3+. This model implies that the mechanisms of luminescence sensitization of Tb3+ and Eu3+ in ZnS nanoparticles are different; namely, Tb3+ acts as a hole trap, whereas Eu3+ acts as an electron trap. Further testing of this model is made by extending the studies from ZnS nanoparticles to other II-VI semiconductor materials; namely, CdSe, CdS, and ZnSe.
Article titled, “X-ray luminescence of LaF3:Tb3+ and LaF3:Ce3+, Tb3+ water-soluble nanoparticles” by Yuanfang Liu in J. Appl. Phys. 103, 063105 (2008) reports x-ray luminescence from LaF3:Ce3+,Tb3+ and LaF3:Tb3+ water-soluble nanoparticles. The x-ray luminescence is dominated by emission from Tb3+ ions, similar to photo luminescence spectra of the nanoparticle aqueous solutions and spectra from nanoparticle powders precipitated from the aqueous samples. Coating the nanoparticles with an insulating inorganic LaF3 or organic H2N—(CH2)10-COOH layer can enhance the x-ray luminescence from the aqueous nanoparticles.
Article titled, “LaPO4:Ce,Tb and YVO4:Eu nanophosphors: Luminescence studies in the vacuum ultraviolet spectral range” by V. Pankratov in JOURNAL OF APPLIED PHYSICS, 110, 053522 (2011) reports comparative analysis of the luminescent properties of nanocrystalline LaPO4:Ce, Tb and YVO4:Eu luminescent materials with macrocrystalline analogues, commercially produced by Philips, has been performed under excitation by pulsed vacuum ultraviolet (VUV) synchrotron radiation, ranging from 3.7-40 eV. Special attention was paid to VUV spectral range, which is not reachable with commonly used lamp and laser sources.
US 2012/0288949 A1 relates to a method for determining the presence or amount of a compound in a sample by interparticle distance-dependent sensing, comprising:    (a) contacting the sample suspected of containing the compound with rare earth doped metal oxide nanoparticles; and    (b) detecting the compound by determining the change in luminescent properties of the rare earth doped metal oxide nanoparticles upon contact with the sample.Article titled, “Trace Explosives Detection by Photoluminescence” by E. Roland Menzel in The Scientific World JOURNAL (2004) 4, 55-66 reports a general lanthanide-based photoluminescence approach which shows promise and the ability to photoluminescence-detect trace explosives in the presence of intense background color and/or background fluorescence by time-resolved imaging.
CN 102071027 A discloses water-soluble rare-earth terbium ion-doped cerium fluoride nanocrystallines and a preparation method thereof.
CN 101864298 A discloses a two rare earth complexes doping Ag@SiO2 fluorescent nanoparticles, characterized in that the fluorescent nanoparticles to double rare earth complexes Eu3+/Tb3+-PABA-DTPA-APTMS silver doped core, the core surface is covered with mesh silica-like structure, with an active surface in Guangxi dioxide amino groups, which double rare earth complexes Eu3+/Tb3+-PABA-DTPA-APTMS mass ratio of silver is: 1:0.176 to 0.2; kernel dioxide silicon mass ratio: 1:5 to 12, and each mg containing 595-630 nmol nanoparticle group. Article titled, “A Strategy to Protect and Sensitize Near-Infrared Luminescent Nd3+ and Yb3+: Organic Tropolonate Ligands for the Sensitization of Ln3+-Doped NaYF4 Nanocrystals” by Stéphane Petoud in J. Am. Chem. Soc., 2007, 129 (48), pp 14834-14835 reports a strategy to sensitize and protect near-infrared (NIR) emitting Nd3+ and Yb3+.
The reported prior arts have drawbacks like costly process, high detection level and multi-step method. Therefore it is the need to develop an easier, quick and effective method for detection of nitro containing compounds preferably explosives with low detection level.